1) Field of the Invention
The present invention relates to a technology for determining a biotic sleep state.
2) Description of the Related Art
There is conventionally known a biotic sleep state determining apparatus that determines a sleep state of a subject based on pulse interval data which is data on one cycle of a pulse wave of the subject, and on body movement data which indicates a body movement of the subject. This sleep state determining apparatus attracts attention as one that can determine a sleep state handy in an ordinary life, as compared with a complicated apparatus, referred to as “polysomnogram”, that determines a sleep state from biological signal patterns such as brain waves, eye movement, an electrical activity of muscles, and an electrical activity of a heart.
In this sleep state determining apparatus, a beat interval of heartbeat that is an activity of an autonomic nerve during sleep is regarded as a pulse interval of a pulse wave, and the sleep state is determined based on autonomic nerve indexes acquired from a fluctuation in the pulse interval. For example, a pulse wave that is a bloodstream fluctuation of a blood vessel of a hand fluctuates synchronously with the heartbeat, so that the beat interval of the heartbeat can be acquired from the pulse interval of the pulse wave. According to conventional techniques disclosed in, for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-291710 and JP-A No. H07-143972, the sleep state is determined based on autonomic nerve indexes acquired from frequency spectrum components of pulse wave data. That is, a series of pulse interval data are acquired from the pulse wave data, and the pulse interval data is converted into a frequency spectrum distribution. The autonomic nerve indexes are acquired from power spectra in a low-frequency region (near 0.05 to 0.15 hertz) and a high-frequency region (near 0.15 to 0.4 hertz) obtained from the series of pulse interval data converted into the frequency spectrum distribution. The sleep state is determined based on the acquired autonomic nerve indexes. According to a conventional technique disclosed in JP-A No. 2002-34955, not only pulse wave data but also body movement data are measured, so as to determine a sleep state as to whether the subject is in a state of awakening, rapid eye movement (REM) sleep, non-rapid eye movement (NREM) sleep, or a difficulty in remaining asleep through the night (intermediate arousal).
The conventional techniques disclosed in the above applications have, however, the following disadvantages. Although the sleep state such as the state of awakening, REM sleep, NREM sleep, or intermediate arousal can be determined, the determination tends to be influenced by a body movement such as movement of the limbs and suffers low determination accuracy for determining the sleep state. This is because the pulse wave data is obtained by measuring pulse waves that are bloodstream fluctuations of the blood vessel of the hand. In addition, the pulse wave data tends to be influenced by a pulse wave abnormality such as arrhythmia or apnea, and suffers the low determination accuracy.